Laser chemical vapor deposition (CVD) is a method for locally depositing thin films onto surfaces without photomasking the surfaces. The method entails focusing a laser beam, typically to micrometer dimensions, on a substrate in the presence of a chemically reactive ambient gas, liquid or a solid film. The laser induces a highly localized chemical reaction resulting in thin film deposition in the laser-treated zone.
The techniques of laser activated chemistry are diverse and can involve pyrolysis (e.g., thermal decomposition) or photolysis (linear photochemistry). In many cases the laser CVD reaction has a very strong nonlinear dependence on laser flux and may be difficult to control, although this same sensitivity to laser flux can be used to achieve greater spatial resolution on the substrate.
There are several ways to implement substrate patterning to achieve laser microchemical reactions. For example, in the laser direct writing geometry, the laser is scanned relative to the substrate to serially write lines. In laser projection patterning, an image field is projected onto the substrate much like a photographic printing method.
U.S. Pat. No. 4,340,617 to Deutsch et al. describes the laser direct writing geometry using photolytic processes in several organometallic precursor vapors. For example, photolysis of dimethyl cadmium or trimethyl aluminum is accomplished using a 257 nm (ultraviolet) frequency-doubled argon-ion laser. This approach was subsequently extended in U.S. Pat. No. 4,868,005 to the photolysis of chromyl chloride and cobalt carbonyl with visible laser light. Photolysis as described by Deutsch et al. is a "substantially linear" process with variation of laser flux.
The alternative approach of pyrolytic deposition, while retaining the laser direct writing geometry, is described by Hanabusa et al. (Appl. Phys. Lett. 35 (8) pp 626-627, October 1979). In this case the scanned focused beam is retained; however, the laser is used to induce a strong local temperature rise in the substrate. Deposition is by pyrolysis, i.e., by thermal decomposition of the organometallic precursor vapor. Pyrolysis is a thermally activated process characterized by a strongly nonlinear (exponential) dependence on temperature.
In the approach of Deutsch et al., the "substantially linear" behavior of the photochemical process allows relatively easy control of the process. That is, a 10% change in laser flux typically results in approximately a 10% change in film thickness. Furthermore, since the laser light is directly coupled into the precursor molecule itself, substrate properties are largely unimportant. In the pyrolytic approach of Hanabusa et al., however, all substrate variations, such as changes in reflectivity or local thermal conductivity, are important. In addition, the result of a small variation in laser power, for example, from a defocus of the lens or from a substrate reflectivity change, results in a much magnified variation in resulting deposition.
In practice, pyrolytic deposition processes have a relatively narrow region of operation in laser power. Writing at constant power across a substrate with variable reflectivity or thermal conductivity can be difficult or impossible since the fixed laser power will be subthreshold for one area while it is damaging elsewhere on the substrate. Furthermore, attaining uniform coverage over steps in substrate height is a critical problem since a constant-power, constant-velocity scan across an irregular topography leads to line thinning and, often, to an open circuit at the step.
In addition, a practical limitation for laser direct instrumentation is the need to meticulously maintain focus over substrates with height changes. At high line resolution the depth of focus for the lens is typically a few micrometers or less, while substrates (particularly in the new fields of microelectromechanics and advanced electronic packaging) may have topography changes of ten to one hundred micrometers or more.
Therefore a method and apparatus is needed to determine a three-dimensional, rather than just a two-dimensional, trajectory for the laser spot. It is highly desirable that this method be able to compensate for local slope changes, reflectivity changes and thermal conductivity changes in the substrate.
Furthermore, although there are no good preexisting solutions to the complex trajectory problem for laser direct write deposition, there is an analogous problem for scanning electron-beam lithography systems. For electron-beam exposure one must compensate for so-called "proximity effects" due to interaction between nearby areas of a lithographic pattern. Proximity effect correction algorithms are the subject of numerous patents and papers in the literature (see, for example, the proceedings of the 34th international symposium on electron, ion and photon beams, (J. Vac. Sci. Technol. B, 8, (1990) pp. 1321-2053).
Although the electron beam system provides a model for an approach to the "proximity effect" problem, the laser direct write deposition problem is distinctly different. There is no need to generate three-dimensional trajectories for electron beams since electron optics have very large or, for practical lithography applications, infinite depth of focus. Additionally, the laser direct write method requires independent control of actual beam intensity and actual scan speed and must consider substrate optical and material properties.